There is known in the art a drive of a compression-distraction apparatus (cf., U.S.S.R. Inventor's Certificate No. 848,011, cl. A 61 B 17/18, published in the Bulletin of Inventions, No. 27, 1981), comprising a lead screw, a gear mounted thereon and, series-connected with said gear via reduction gear, a ratchet wheel, a lever with a pawl and an electromagnet interacting with one of the lever arms. Said prior art device is further provided with a power pack connected to an electromagnet.
The afore-described drive provides for measured and atraumatic micromovement of bone fragments. However, said prior art drive suffers from excessively large mass and dimensions whereby its clinical use is rather difficult.
This disadvantage is primarily due to the low specific force per unit weight developed by the drive.
Another prior art drive of a compression-distraction apparatus appears more improved (cf., U.S.S.R. Inventor's Certificate No. 865,284, cl. A 61 B 17/18, published in the Bulletin of Inventions, No. 35, 1981) and provides, analogously with the foregoing prior art drive, for atraumatic movement of bone fragments that is close to natural physiological growth. Said latter prior art drive of compression-distraction apparatus comprises a lead screw mated by threading with an opening of a ratchet wheel rotatably positioned in a housing. Interacting with teeth of the ratchet wheel is a pawl coupled with a load-bearing element manufactured from an alloy possessing plastic memory and having an electric heater. The ratchet wheel of said prior art drive has internal mesh while the pawl is positioned on the smaller arm of a lever arranged inside the wheel and lying in the same plane therewith. The load-bearing element is fashioned as a shackle arranged below the lever in a parallel plane therewith and connected by its one end with the housing wall while its other end is connected with the long arm of the lever and with a resilient member serving to keep the load-bearing element in a bent position. The electric heater with which the load-bearing element is provided is connected to a power pack.
The last-described prior art unit, while exhibiting the same distraction force as the previously described electromagnetic drive, features considerably smaller dimensions and weight. However, attempts at expanding the range of clinical application of said latter unit have shown that the force it develops is insufficient for ensuring reliable operation. Thus, an increase of the distraction force above 300-350 N leads to failures in the drive operation caused by the fact that the load-bearing element reaches the breaking stress. Actual loads in the course of the drive operation may exceed considerably those cited above in view of the functional load on an extremity reaching, as a maximum, the magnitude of the patient's weight. Therefore, it is very important to find a solution to the problem of ensuring an adequate distraction force.
The distraction force developed by the prior art drive may be increased by way of increasing the cross-sectional area and dimensions of the load-bearing element. However, an increase of thos dimensions results in an increase of the ratchet wheel diameter, housing diameter and of the overall weight of the drive. Second, an increase of the cross-sectional area and dimensions of the load-bearing element involves a direct increase in the consumption of energy required for heating the element to the operation temperature, which affects considerably the duration of autonomous supply of the drive.
Therefore, the prior art drive may only develop a distraction force required for its clinical use only on condition of a considerable increase of the drive dimensions and weight. This restricts considerably the range of clinical application of the unit and, in a number of cases, e.g., in apparatuses for elongation of short tubular bones (phalanges, metacarpal bones), makes it problematic. Moreover, a rise in the energy intensity of the drive resulting from an increase of the cross-sectional area and dimensions of the load-bearing element reduces the autonomous supply of the drive and further restricts the range of clinical application of the prior art unit.